1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus, and more particularly to a discharge lamp lighting apparatus to light a plug of discharge lamps.
2. Description of the Related Art
A liquid crystal display (LCD) apparatus as a flat panel display apparatus is used in various applications. Since a liquid crystal in the LCD apparatus does not emit light by itself, a lighting device is required separately in order to achieve a good display. A backlight device to light a liquid crystal panel from behind is one type of lighting device. The backlight device uses primarily a cold cathode lamp as a discharge lamp and incorporates a discharge lamp lighting apparatus including an inverter to drive the cold cathode lamp.
Recently, the LCD apparatus is becoming larger and larger for use in, for example, a large-screen TV, and therefore a plurality of discharge lamps are used in a backlight device in order to achieve sufficient screen brightness for the LCD apparatus. In such a backlight device, if brightness varies from one discharge lamp to another, the display screen of the LCD apparatus incurs non-uniformity thus significantly degrading the display quality. So, not only high luminance of each discharge lamp but also uniformity in brightness of all the discharge lamps is required. Further, cost reduction of the discharge lamp lighting apparatus is strongly requested due to the price reduction of the LCD apparatus.
The brightness variation over the discharge lamps can be prevented by equalizing lamp currents flowing through respective discharge lamps. The lamp currents can be equalized by such a method that transformers that are provided in a number equal to the number of the discharge lamps are individually controlled by respective control ICs. This approach, however, requires an increased number of components thus pushing up cost, which eventually results in an increased cost of the discharge lamp lighting apparatus.
The lamp currents can alternatively be equalized by providing balance coils, but this alternative approach requires a large number of balance coils for multiple discharge lamps, and the balance coils must be designed individually with different specifications because the values of currents flowing through the balance coils differ from one another depending on the places where the balance coils are disposed. Consequently, the number of components is increased pushing up the cost on the discharge lamp lighting apparatus.
A discharge lamp lighting apparatus is proposed (refer to, for example, Japanese Patent Application Laid-Open No. H11-260580) as still another approach. In the discharge lamp lighting apparatus, inductance values are controlled by variable inductance elements, rather than balance coils, so as to control respective lamp currents and reduce the variation in brightness of the discharge lamps for uniform brightness over the display screen.
FIG. 5 is a circuitry of the discharge lamp lighting apparatus which is disclosed in the aforementioned Japanese Patent Application Laid-Open No. H11-260580, and in which two discharge lamps are provided.
Referring to FIG. 5, field effect transistors (FETs) 102 and 103 as switching elements are connected in series between the positive and negative electrodes of a DC power supply 101, and the connection portion of the source terminal of the FET 102 and the drain terminal of the FET 103 is connected to the negative electrode of the DC power supply 101 via a series resonant circuit 120A which includes a capacitor 122a and a winding 121a of an orthogonal transformer 121A constituting an variable inductance element, and also via a series resonant circuit 120B which includes a capacitor 122a and a winding 121a of an orthogonal transformer 121B constituting an variable inductance element.
The connection portion of the winding 121a of the orthogonal transformer 121A and the capacitor 122a is connected to the negative electrode of the DC power supply 101 via a series circuit including a capacitor 110a, a discharge lamp 111a, and a current detecting resistor 123a of a control circuit 123A, and an output signal of the control circuit 123A is fed to a control winding 121b of the orthogonal transformer 121A.
The control circuit 123A supplies a control current to the control winding 121b of the orthogonal transformer 121A, and is arranged such that the connection portion of the discharge lamp 111a and the current detecting resistor 123a is connected to the inverting input terminal of an operation amplifying circuit 123c via a rectifier diode 123b, the connection portion of the rectifier diode 123b and the inverting input terminal of the operation amplifying circuit 123c is connected to the negative electrode of the DC power supply 101 via a smoothing capacitor 12d, the non-inverting terminal of the operation amplifying circuit 123c is connected to the negative electrode of the DC power supply 101 via a battery 123e having a reference voltage Vref to determine a reference value of a current of the discharge lamp 111a, and that the output terminal of the operation amplifying circuit 123c is connected to the negative electrode of the DC power supply 101 via the control winding 121b of the orthogonal transformer 121A.
The control circuit 123A functions to control the current of the discharge lamp 111a. Specifically, the control circuit 123A operates such that when the current of the discharge lamp 111a is to be increased, the control current of the control winding 121b of the orthogonal transformer 121A is increased so as to decrease the inductance value of the winding 121a of the orthogonal transformer 121A thereby increasing the resonant frequency f0 the series resonant circuit 120A thus decreasing the impedance of the series resonant circuit 120A at a driving frequency consequently resulting in an increase of a voltage generated across the both ends of the capacitor 122a, and such that when the current of the discharge lamp 111a is to be decreased, the control current of the control winding 121b of the orthogonal transformer 121A is decreased so as to increase the inductance value of the winding 121a of the orthogonal transformer 121A thereby decreasing the resonant frequency f0 the series resonant circuit 120A thus increasing the impedance of the series resonant circuit 120A at a driving frequency consequently resulting in a decrease of a voltage generated across the both terminals of the capacitor 122a. 
There is provided another circuit which includes the orthogonal transformer 121B, and which is constituted identically and functions identically with the above-described circuit including the orthogonal transformer 121A.
In the discharge lamp lighting apparatus shown in FIG. 5, the currents flowing through the discharge lamps 111a and 111b are controlled at a predetermined value while a switching frequency of a control signal to be supplied from a control circuit 104 to the FETs 102 and 103 is set at a fixed value without a switching frequency control, thus uniform brightness between the discharge lamps 111a and 111b is achieved without performing a complicated frequency control at the control circuit 104.
A high voltage of about 1,500 to 2,500 V is required to turn on a cold cathode lamp, and a voltage of about 600 to 1,300 V must be applied to keep the cold cathode lamp lighted on. Accordingly, a power supply to supply such a high voltage is required in a discharge lamp lighting apparatus. Since the discharge lamp lighting apparatus shown in FIG. 5 is not provided with a step-up circuit, the DC power supply 101 outputs a high voltage in order to duly turn on the discharge lamps 111a and 111b. 
Also, since the FETs 102 and 103 to turn on the discharge lamps 111a and 111b, and the control circuit 104 to control the FETs 102 and 103 are connected to the DC power supply 101 to output a high voltage, the FETs 102 and 103 and the control circuit 104 must be composed of high withstand voltage materials which are expensive, thus pushing up the cost of the components, and eventually the cost of the apparatus.
Further, in the discharge lamp lighting apparatus shown in FIG. 5, the capacitors 110a and 110b, which are current controlling capacitors (so-called “ballast capacitors”) to stabilize the lamp currents of the discharge lamps 111a and 111b, are connected in series to the discharge lamps 111a and 111b, respectively, and a high voltage is applied to the capacitors 110a and 110b. Consequently, the capacitors 111a and 110b must also be composed of high withstand voltage materials, and since the current controlling capacitors must be provided in a number equal to the number of discharge lamps to be driven, the coat of the apparatus is pushed up definitely. Also, since a high voltage is applied to the capacitors 110a an 110b an described above, there is a problem also in terms of component safety.
Further, in the discharge lamp lighting apparatus shown in FIG. 5, since the lamp current is controlled by a variable inductance element only, a sufficient variation range must be secured for the variable inductance element in order to duly control the lamp current. Thus, the variable inductance element must be increased in dimension so as to get its maximum inductance value increased. However, if such a discharge lamp lighting apparatus is incorporated in, for example, a backlight device for a low-profile TV, components in the apparatus are forced to have a limited height from a printed board, which makes it difficult to increase the dimension of the variable inductance element to be mounted on the printed board.
And, since impedance is increased with an increase of inductance, when the maximum inductance value of the variable inductance element is increased, it is necessary to increase also a voltage to be supplied to the discharge lamp via the variable inductance element. Accordingly the load of the DC power supply 101 to output a high voltage is increased, and the loads of elements constituting the FETs 102 and 103 and the control circuit 104 to light the discharge lamps 111a and 111b are also increased. Consequently those components must be composed of high withstand voltage materials which are expensive, thus pushing up the cost of the components, and eventually the cost of the apparatus.